Earlier, he had imposed the "heaviest-ever" sanctions on North Korea's shipping companies in a bid to prevent the reclusive nation from acquiring nuclear weapons and developing intercontinental ballistic missiles.

"If the sanctions don't work, we'll have to go phase two. And phase two may be a very rough thing. May be very, very unfortunate for the world," Trump told reporters at a joint news conference with the visiting Australian Prime Minister Malcolm Turnbull.

However, he hoped that the sanctions will work.

"We have tremendous support all around the world for what we're doing. It really is a rogue nation," he said.

"If we can make a deal, it'll be a great thing. If we can't, something will have to happen. So we'll see," the US President said responding to a question on North Korea.

Earlier, the Trump Administration announced a series of sanctions the largest so far against the authoritarian North Korean regime.

"North Korea -- we imposed today the heaviest sanctions ever imposed on a country before," Trump said in his remarks to the Conservative Political Action Conference.

"Frankly, hopefully something positive can happen. We will see," he hoped.

"This action targets the deceptive shipping practices that have enabled the Kim regime to fund its dangerous weapons programmes," US Treasury Secretary Steven Mnuchin told reporters at a White House news conference.

The latest American actions target shipping and trade companies, vessels and individuals across the world who are working on North Korea's behalf, he said as he announced imposing sanctions against 27 entities, 28 vessels, and one individual.

All of them are involved in sanctions evasion schemes, he said.

These shipping and trade companies, vessels and one individual, totaling 56 designations are located, registered or flagged in countries all over the world, including North Korea, China, Singapore, Taiwan, Hong Kong, Marshall Islands, Tanzania, Panama and Comoros.

"Today's actions will significantly hinder North Korea's ability to conduct evasive maritime activities that facilitate illicit coal and fuel transports, and limit the regime's ability to ship goods through international waters," Mnuchin said.

The American actions are part of the ongoing maximum-economic-pressure campaign to cut off sources of revenue that this regime derives from UN- and US-prohibited trade to fund its nuclear and ballistic missile programs, he added.

The US is also issuing a global shipping advisory, in conjunction with the Coast Guard and the State Department, to put everyone on notice of North Korea's illicit maritime tactics, and underscore the significant sanctions risk of engaging in maritime business with North Korea, Mnuchin said.

"We are releasing new imagery of the deceptive shipping practices used by those who aid and profit from illicit trade with North Korea. These images from December 2017 reveal ship-to-ship transfers of fuel and other products destined for North Korea in an attempt to evade sanctions," he said.

They shine a spotlight on the practices employed by the government of North Korea to falsify identifying information on ships, and conceal illicit cargo, Mnuchin said.

These sanctions evasion tactics are prohibited by UN Security Council resolutions, and we are fully committed to shutting down those who engage in trade with them, he asserted.

"Through today's actions, we are putting companies and countries across the world on notice that this administration views compliance with US and UN sanctions as a national security imperative. Those who trade with North Korea do so at their own peril," Mnuchin said.

The Treasury Secretary warned that the countries who do business with North Korea will not be able to do business with the US.

A senior administration official told reporters that these designations are a critical part of our maximum-pressure campaign to diplomatically and economically isolate North Korea.

"These actions represent the latest development in the coordinated, whole-of-government US effort to disrupt, deter and dismantle North Korea's illicit maritime shipping activities that Pyongyang uses to skirt UN sanctions, and to fund its WMD (programs and delivery systems," the official said.